The objective of these studies is to investigate antigens of possible significance to the pathogenesis of rheumatoid arthritis (RA). Antisera have been made in rabbits to RA-derived synovial fibroblasts cultured in vitro and specificities of the antisera determined. A common reactivity of the antisera has been shown in human serum, fetal calf serum, RA and non-RA synovial fluids, and RA and normal synovial and dermal fibroblast extracts. However, the human serum, synovial fluid, and fibroblast extracted antigens all have different electrophoretic mobilities. This antigen may be a secreted product of the fibroblast found in human serum. Studies are being done to see if any portion of the reactivity is specific to the RA synovial fibroblast and thus can be used as a serum marker for synovial tissue inflammation and lining cell hyperplasia. Anti-RA synovial fibroblast sera absorbed with whole human or fetal calf serum retains an antibody reactivity to a fibroblast membrane-associated antigen as shown by indirect immunofluorescent assays performed on living cells grown and reacted on slides. The absorbed antisera are being further tested and absorbed to determine differences of membrane-associated antigens of RA versus normal, and synovial versus dermal fibroblasts. The antisera to RA synovial fibroblasts also reacts with a component in RA synovial fluid after immunoelectrophoresis, forming an arc of precipitation not found in human serum or non-RA synovial fluids. Further tests will be done to determine the frequency of this antigenic reactivity in synovial fluids and its significance in relation to RA synovial inflammation. Antibody synthesized in vitro by RA synovial tissue has been isolated by affinity chromatography and characterized immunochemically. Binding of radiolabelled antibody to tissue cultured fibroblast cells has been demonstrated and the nature and specificities of these reactivities will be investigated.